Introduction to Frame Relay

Frame Relay is a long distance, non-persistent connections Layer 2 protocol, that allows several data connections(called virtual circuits) to be multiplexed onto a single physical link. The connection to the network edge is often a leased line but dial-up connections are available from some providers using ISDN or xDSL lines. Frame Relay implements no error or flow control, it relies on upper-layer protocols for error correction instead. Frame Relay specifies only the connection between a router and a service provider’s local access switching equipment. The data transmission within the service provider’s Frame Relay cloud is not specified. A connection identifier is used to map packets to outbound ports on the service provider’s switch. When the switch receives a frame, a lookup table is used to map the frame to the correct outbound port. The entire path to the destination is determined before the frame is sent.

Most Frame Relay functions exist at the lower two layers of the OSI Reference Model. Frame Relay is supported on the same physical serial connections that support point-to-point connections. Cisco routers support the following serial connections: EIA/TIA-232, EIA/TIA-449, V.35, X.21, EIA/TIA-530. Upper-layer information (such as IP data) is encapsulated by Frame Relay and is transmitted over the link.

frame relay

Understanding the following terms is import to understand how the Frame Relay works:

• Virtue Circuit (VC) -- a logic connection between endpoints, more specifically, a logical connection from one DTE through the WAN cloud to the DTE at the other end of the network. There are two types of VC, Switched Virtue Circuit (SVC) and Permanent Virtue Circuit (PVC). SVC is established on-demand. PVC is similar to leased line, it don't need circuit establishment or teardown but more expensive.


• Data Terminal Equipment (DTE) is an end instrument that serves as a data source or a data sink. DTE devices are typically owned by your organization. The most common DTE is a router.


• Data Communications Equipment (DCE) is a device that sits between the DTE and a data transmission circuit. The DCE converts user data into the service provider’s preferred format, DCE may be a part of the DTE or intermediate equipment. A DCE is typically a switch inside a service provider’s network.


• Access Link -- is the local loop between the DTE and the DCE.


• Access Rate or Committed Information Rate (CIR)-- the minimum data transfer rate the ISPs guaranteed to the service subscriber. However, because the physical circuits of the Frame Relay network are shared amongst many subscribers, there will often be excess bandwidth available at any point in time. This allows a customer’s traffic to actually “burst” to higher speeds, as available network bandwidth permits.


• Data Link Connection Identifier (DLCI) -- A number to identify the connection or virtue circuit between a router and the destination. It is the address field in a frame-relay header.


• Non-Broadcast Multiaccess (NBMA) -- one line have multiaccess to multiple different devices, but not broadcast.


• Local Management Interface (LMI) -- A signaling standard used to manage the connection between the DTE (usually a router) and DCE (the frame relay switch). LMIs track and manage keepalive mechanisms, multicast messages and status. If here's an LMI mismatch, the physical interface will remain up but the line protocol will go down. LMI can be configured (in Cisco IOS 11.2 and later), but routers can autosense LMI types by sending a status request to the Frame Relay switch. The router configures itself to match the LMI type response. The three types of LMIs supported by Cisco Frame Relay switches are Cisco (developed by Cisco, StrataCom, Northern Telecom, and DEC), ansi Annex D (ANSI standard T1.617), and q933a (ITU-T Q.933 Annex A).

When LMI Autosense is in operation, the DTE will send three LMI Status messages. The DCE will respond with one LMI Status message. The DTE sees that Status message coming in, and from that point on only sends the LMI Status message type that matches the type it received from the DCE.

LMI autosense

To debug the frame relay network, use command "show frame lmi" and "debug frame lmi".

Inverse ARP enables dynamic Frame Relay mapping. Inverse ARP is often disabled in today's networks, and this is done by running the command "no frame-relay inverse-arp" at the interface level.